Testing - Concrete Mix Design, Quality Control and Specification - page 169

that early-age strength will increase with increasing supply temperature,
the additional maturity being sufficient to more than offset any increased
water requirement. This is more likely to occur with say a 3-day test than a
7-day test and in cold climate countries rather than hot ones.
The temperature of the test specimens prior to demolding can have
a significant effect, particularly on early-age strength. This is the rea-
son that a properly controlled and managed testing facility on site is so
important.
7.5.8 Moisture content
Earlier moisture probes for real-time assessment of moisture content of
aggregate suffered from problems with clumping of aggregate around the
probe. However, recent microwave-based probes with suitable calibration
do appear to provide a good estimate of moisture content in aggregates
during production.
In the third edition, Day suggested that with the low cost and ready avail-
ability of microwave ovens there should be an increasing use of measuring
moisture content by drying a sample of wet concrete taken back to the
laboratory. AASHTO T318 “Standard Method of Test for Water Content
of Freshly Mixed Concrete Using Microwave Oven Drying” is a useful
test for monitoring the water content of the fresh concrete. The procedure
takes about 15 minutes and the single operator within-laboratory standard
deviation has been found to be 1.6 kg/m 3 . This procedure is particularly
useful in controlling questionable premix suppliers. The Port of New York
and New Jersey uses this procedure as a quality control procedure on its
projects. Although the largest source of error should be in a nonrepresen-
tative ratio of mortar to coarse aggregate in the sample, the AASHTO
procedure seems to provide reasonable repeatability without sieving.
7.5.9 Wet analysis
The UK RAM (rapid analysis machine) is an apparatus designed by CACA
to split a sample of fresh concrete into its constituent parts. It is well known
but apparently little used outside the United Kingdom. According to Neville
(2011), the RAM has not proved successful, and the ASTM test methods for
the fresh cement and water content determination have been withdrawn.
Clearly the increased complexity of the cementitious component in concrete
due to the use of binary and ternary blends makes rapid wet analysis based
on particle size or even chemistry difficult.
As regards the relative proportions of the dry ingredients, most plants
and projects are served by a computerised batching plant that can provide
a hard copy computer record of the batch weights, which should be able
to settle any question of deliberate deception. Focusing on performance